A Long-Term Evolution (LTE) system offers high peak data rates, low latency, improved system capacity, and low operating cost resulting from simplified network architecture. LTE systems also provide seamless integration to older wireless network, such as GSM, CDMA and Universal Mobile Telecommunication System (UMTS). In LTE systems, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, such as evolved Node-B's (eNBs) communicating with a plurality of mobile stations referred as user equipment (UEs). Dual Connectivity (DuCo or DC) UE is introduced to enhance mobility, bandwidth, and flexibility use of the network. A UE with dual connectivity has more than one transceivers corresponding to more than one MAC entities. The multiple MAC entities can be configured to communicate with multiple eNBs simultaneously.
The upcoming next generation “5G” Millimeter Wave (mmWave) small cell is expected to coexist with microwave (e.g., E-UTRAN) macro cells for a long time. The macro-assisted mmWave cellular systems exploit the fact that mmWave small cells and microwave macro cells may compensate each other very well in coverage area, link capacity, spectrum availability, and service robustness. From both network and radio access's perspectives, mmWave is currently considered a very promising choice for in-door or out-of-door “5G” cellular small cells, which may compensate microwave macro-cell in shortage of spectrum or in need for economical high-speed data services. In particular, the small cells offer downlink (DL) throughput boosting or coverage extension for an umbrella macro-cell at its edge. On the other hand, macro-cell coverage makes up mmWave's directional coverage limitation and bursty link disruption by offering reliable omni-directional overlay services for time-critical or mission-critical control signaling, or offering more robust and seamless services for low-rate high-mobility (voice) users. Together they constitute a layered or scalable communication infrastructure that promise reliability, wide coverage, economical yet diversified mobile QoS services.
The existing LTE HetNet Dual Connectivity (DuCo) architecture is not fine-tuned for mmWave small cells that have new radio characteristics and face new 5G requirements as well. The LTE DuCo architecture is designed only for some less densely deployed, relatively low-frequency microwave smallcell scenarios, and not optimized for stationary or dense scenarios with Gbps mmWave small cells. Furthermore, LTE DuCo mobility is used in omni-directional cellular systems of no beamformed control channel. While LTE DuCo may be used as the baseline for macro-assisted mmW mobility, it lacks some reliability due to beamforming. With UE's control channel anchored solely to master eNB (MeNB), DuCo allows 2-way PDCP bearer split between MeNB and secondary eNB (SeNB) for data plane to UE. UE sees service degradation at the edge due to signal weakness or frequent (beamformed) link disruption during S2S addition/release or otherwise rate mismatch during S2M fallback. Similar 2-way split DuCo concepts are applied to other Multi-RAT systems, e.g., LTE-WiFi (LWA) aggregation, with potentially the same limitation.
Compared to 4G system, 5G demands uniform UE service experience even at “cell edge”, while 5G mmWave-specific CH and BF characteristics present new challenges to mobility reliability and seamlessness. First, directional beamforming makes mobility management even harder and more time critical due to complex, time-consuming beam alignment, beam switching, and beam tracking. Second, Multiple levels of beams, multiple beams per level, multiple (TDM) BF-ed control beams per cell to scan makes scanning more time and power consuming with frequent and intermittent link disruptions and blockage. Third, high handover (HO) ping-pong rate happens even at mild channel blocking while HO failure rate is very high at severe blockage. The service rate and reliability at cell edges of 5G multi-RAT systems, if still following the existing design of DuCo's data plane, may not meet the 5G requirements for uniform edge or center services. For this purpose and for the purpose of providing more robust mobility, a bandwidth aggregation at the cell edge of neighboring small cells and/or macrocell, and a make-before-break mobility scheme, may present a whole solution. Such a design is not yet supported by the DuCo architecture in either user plane or control plane. Therefore, an enhanced macrocell-assisted smallcell mobility with multi-connectivity, for example for a multi-RAT system including 5G enhanced LTE macro plus (mmWave) smallcell systems, or for a LTE-WiFi aggregation systems, is desired to meet both the 5G demands and the mmWave mobility challenges.